I. Field of the Invention
The invention relates to communication systems. More particularly, the invention relates to resource allocation in a multiple access system.
II. Description of the Related Art
FIG. 1 is an exemplary embodiment of a terrestrial wireless communication system 10. FIG. 1 shows three remote units 12, 13, and 15 and two base stations 14. In reality, typical wireless communication systems may have many more remote units and base stations. In FIG. 1, the remote unit 12 is shown as a mobile telephone unit installed in a car. FIG. 1 also shows the fixed location remote unit 15 in a wireless local loop system and the portable computer remote unit 13 in a standard cellular system. In the most general embodiment, the remote units may be any type of communication unit. For example, the remote units may be hand-held personal communication system (PCS) units, portable data units such as a personal data assistant, or fixed location data units such as meter reading equipment. FIG. 1 shows a forward link signal 18 from the base stations 14 to the remote units 12, 13 and 15 and reverse link signal 19 from the remote units 12, 13 and 15 to the base stations 14.
In a typical wireless communication system, such as that illustrated in FIG. 1, some base stations have multiple sectors. A multi-sectored base station comprises multiple independent transmit and receive antennas as well as some independent processing circuitry. The principles discussed herein apply equally to each sector of a multi-sectored base station and to a single sectored independent base station. For the remainder of this description, therefore, the term xe2x80x9cbase stationxe2x80x9d can be assumed to refer to either a sector of a multi-sectored base station, a plurality of sectors associated with a common base station or a single sectored base station.
In a CDMA system, remote units use a common frequency bandwidth for communication with all base stations in the system. Use of a common frequency bandwidth adds flexibility and provides many advantages to the system. For example, use of a common frequency bandwidth enables a remote unit to simultaneously receive communication signals from more than one base station, as well as transmit a single signal for reception by more than one base station. The remote unit discriminates the simultaneously received signals from the various base stations through the use of the spread spectrum CDMA waveform properties. Likewise, the base station can discriminate and separately receive signals from a plurality of remote units.
In a wireless system, maximizing the capacity of the system in terms of the number of simultaneous calls that can be handled is extremely important. System capacity in a spread spectrum system is increased if the power received at the base station from each remote unit is controlled such that each signal arrives at the base station receiver at the minimum power level required to obtain a desired signal quality level. If a signal transmitted by a remote unit arrives at the base station receiver at a power level that is too low, the signal quality may fall below an acceptable level. If, on the other hand, the remote unit signal arrives at a power level that is too high, communication with this particular remote unit is acceptable, but the high power signal acts as interference to other remote units. This excessive interference may adversely affect communications with other remote units. Thus, in general, a remote unit located near the base station transmits a relatively low signal power while a remote unit located at the edge of the coverage area transmits a relatively large signal power.
In more advanced systems, in addition to controlling the power level at which the remote unit transmits on the reverse link, the data rate at which the remote unit transmits on the reverse link is also controlled. A remote unit located on the edge of a coverage area may reduce the data rate at which it transmits in order to increase the signal quality of the signal as received at the base station. By reducing the data rate, the time devoted to each bit may be increased, thus, increasing the energy devoted to each bit and increasing the performance of the link.
In addition to link performance, the use of variable data rates can also provide other benefits to the system. For example, a remote unit may generate a stream of data which is being produced at a data rate significantly below a maximum data rate. The remote unit may chose to transmit the data at a rate lower than the maximum rate in order to conserve remote unit power and spectral resources. In addition some remote units may be categorized according to the level of service which they provide. For example, a preferred client remote unit may provide data transfer up to a maximum rate while an economy level remote unit may provide data transfer at one-eighth, one-quarter or onehalf of the maximum rate. A remote unit which transmits at less than the maximum rate may transmit at a lower power level or it may transmit only a portion of the time. For example a remote unit transmitting at one-quarter of the maximum rate may transmit its signal at one-quarter of the power which would be necessary to transmit a full-rate signal Alternatively, a remote unit which is transmitting at one-quarter of a maximum rate may transmit with a duty cycle of approximately one over four. In either case, a remote unit which transmits at less than the full rate generates less interference and consumes less system resources than a remote unit transmitting at full rate, thereby, freeing system resources for use by other remote units.
If a minimum acceptable signal quality is specified, an upper bound on the number of simultaneous users which can communicate through a base station can be calculated at a given level of interference. This upper bound is commonly referred to as pole capacity. The ratio of actual users to pole capacity is defined as the loading of the system. As the number of actual users approaches the pole capacity, loading approaches unity. A loading close to unity implies potentially unstable behavior of the system. Unstable behavior can lead to degraded performance in terms of error rate performance, failed hand-offs, and dropped connections. In addition, as loading approaches unity, the size of the coverage area of the base station shrinks such that users on the outer edge of the coverage area may no longer be able to transmit sufficient power to communicate with the base station at an acceptable signal quality even at the lowest available data rate.
For these reasons, it is advantageous to limit the usage of a system such that loading does not exceed a specified percentage of the pole capacity. One way to limit the loading of the system is to deny access to the system once the loading of the system has reached a predetermined level. For example, if the loading increases above 70% of the pole capacity, it is advantageous to deny requests for additional connection originations and to refrain from accepting hand-off of existing connections. In a system in which the remote units are capable of transmitting at multiple data rates, the loading of the system can also be controlled by controlling the data rate at which the remote units transmit. For a given level of loading, by reducing the data rate at which each remote unit may transmit, the total number of remote units able to access the system may be increased.
In a typical digital data multiple access system, a remote unit establishes a communication session with the base station. The session remains active until power is removed from the remote unit or until the remote unit requests a disconnection. Once a session has been established, a remote unit transmits bursts of data. For example, if a remote unit user connects to an internet via a wireless connection and his notebook computer, he establishes a session when he logs into the network. If the remote unit user generates an e-mail message, the remote unit generates a burst of data when it transfers the e-mail message. The burst of data may contain one or more packets of data. The packets of data typically comprise many wireless link frames of data.
In a system in which the data rate of the remote unit is controlled by the base station, before the remote unit transmits a burst of data, it sends an access request message to the base station. Typically, the access request message specifies the desired transmission data rate. In response the base station may give permission for the remote unit to transmit at the desired data rate, may give the remote unit permission to transmit at a lower data rate, or may deny access to the system. The use of such a system has several drawbacks. For example, the use of an access request message consumes the precious reverse link resources. In addition, the transmission of data over the reverse link is delayed while the remote unit and base station negotiate a data rate. In addition, the algorithm which must be used by the base station to respond to the access request messages from a plurality of remote units is complicated and consumes considerable base station resources.
For these reasons, there has been a long felt need in the industry for a method and apparatus for controlling access to a multiple access system employing a variable data rate transmission scheme.
A base station is used to control the reverse link transmission rates for remote units within the corresponding coverage area. The base station monitors the reverse link loading and dynamically adjusts the transmission rate set point. The transmission rate set point may be defined in terms of a maximum transmission rate and a transmission probability. The maximum transmission rate defines the maximum reverse link data rate available to the remote units. The transmission probability is used to control the probability that a remote unit transmits at the given maximum transmission rate. The base station may broadcast the transmission rate set point to the remote units. The remote units may transmit at a rate lower than the maximum transmission rate at any time. In this way, the loading of the system is controlled in a fast and stable manner which efficiently utilizes available system resources.